The basic unit for digital logic circuit operation can be viewed as an inverter comprising a power supply charging a capacitor through a load device in series with a switching device activated by an input signal. The output of the circuit connects to the node between switching device and load device. The aforementioned capacitor is the node capacitance which comprises the switching device capacitance, the input capacitance of the next stage, and parasitic capacitances of the node, such as stray capacitances. With the load device connected to the high terminal of the power supply, and one of the switching device terminals grounded, the capacitor is shorted to ground when the switching device is conducting, and the capacitor is charged to the power supply voltage when the switching device is non-conducting. The stationary current level drawn by the circuit when the switching device is conducting is governed by the power supply voltage and the load resistance. Thus a large load resistance is desirable for achieving low power consumption in this state. When the switching device is turned off, the capacitor charges to the power supply voltage by current flow through the load device. A small load resistance is required to achieve fast switching speed from the on-state to the off-state of the switching device. Thus there are two opposing requirements, a large load resistance for low power consumption, and a small load resistance for fast switching speed, for low power-high speed digital circuits.
These requirements have been met in silicon-based complementary circuits by using as load a second transistor of opposite polarity type, whose resistance is modified by the input signal applied to the switching transistor: the load transistor is turned off when the switching transistor is turned on, and vice versa.
In the quest for higher switching speed, n-channel gallium arsenide field effect transistors have proven superior to silicon transistors because the mobility of electrons in gallium arsenide is larger than that in silicon. Also, the saturation drift velocity of electrons in strong electric fields is larger in gallium arsenide than it is in silicon. Unfortunately the mobility of holes is smaller in gallium arsenide than in silicon, so that complementary gallium arsenide circuitry is unattractive because of the slowness of the p-channel device. Accordingly, ultra-fast gallium arsenide switching circuits have been built using either a fixed resistor load or an n-channel fixed transistor load device. This eliminates the possibility of turning the load device off by the input signal which turns the switching device on, and vice versa. An up-to-date description of gallium arsenide fast switching circuits can be found in several papers in the Special Issue on Microwave Devices, No. 6, Vol. ED-25 of the IEEE, Transactions on Electron Devices, June 1978, pp. 559-639. The performance of these circuits is being characterized by the speed-power product. The speed of GaAs-transistors is adequate for fast switching of these circuits from the off-state of the transistor to the on-state of the transistor. During this switching the node capacitance is discharged by current flow through the transistor. However, the switching speed of the circuit from the on-state of the transistor to the off-state of the transistor is governed by the charging of the node capacitance through the load device. To achieve high speed at low power, all these circuits face the afore-mentioned dilemma of the two opposing requirements on the magnitude of the load resistance.
It is an objective of this invention to provide a switching circuit having a load device of large resistance under steady state current flow, but smaller resistance during most of the transient from the on-state with current flow to the off-state without current flow.
It is a further objective of my invention to describe an integrated ultra-high speed switching circuit of low power consumption based on the above-mentioned principle.
It is a further objective of my invention to describe an ultra-fast gallium arsenide integrated switching circuit of low power consumption.
It is a further objective of this invention to describe an ultra-fast gallium arsenide integrated switching circuit activated by a light beam with means for fast recovery after the light beam is switched off.